For a weighing device for measuring a weight of an article, a weight sensor such as a load cell of strain gauge type and a force balance is employed. Such a weight sensor as cited has a natural frequency which is subject to a gross weight including a weight of an article itself and a tare. When external vibration at frequencies in a band including a natural frequency acts upon a measuring system, vibration at a frequency close to the natural frequency in the external vibration is amplified, to work as a vibration noise, which is then introduced in a measuring signal output from a weight sensor. Such a vibration noise as noted above will be hereinafter referred to as a “natural frequency noise”.
Also, in measuring a load of a transfer system, a noise caused due to vibration of a rotation system such as a motor for driving a conveyor belt and a transfer roller, or an electrical noise associated with a commercial power supply, works as a vibration noise, which is then superimposed on a measuring signal.
It is noted that Patent document 1 describes techniques regarding a weighing device, and Patent document 2 describes techniques regarding a digital filter. Further, Non-patent document 1 describes a solution to an optimization problem.
Patent document 1: Japanese Patent Application Laid-Open No. 2004-150883
Patent document 2: Japanese Patent Application Laid-Open No. 2007-129408
Non-patent document 1: J. F. Sturm, “Using sedumi 1.02, A MATLAB toolbox for optimization over symmetric cones (Updated for Version 1.05)”, Optimiz. Methods and Syst. II, 1999, Vol. 11-12, pp. 625-653
As described above, since a natural frequency of a weight sensor is subject to a weight of an article and a tare, the frequency or the level of a natural frequency noise changes as the weight of the article or the tare changes. Conventionally, a tare, e.g., a weight of a conveyor, has been a predominating factor, and thus, change in the frequency or level of a natural frequency noise which is caused by change in the weight of the article has been negligible. However, in recent years, with a demand for more accurate measurement of a weight of a lighter article, there is a trend toward a smaller tare, so that such change in the frequency or level of the natural frequency noise as noted above has become too significant to neglect.
Further, the frequency or level of a vibration noise associated with a motor or a transfer roller changes in accordance with specifications of a transfer system such as a transfer speed, for example. For this reason, it is desired to design a digital filter which is capable of easily canceling a vibration noise even if the frequency or level of a vibration noise changes due to change in the type or size of an object under measurement, or change in specifications of a transfer system.